1. Field of the Invention
This invention relates to an image forming apparatus, such as a copier, a printer or the like, and more particularly, to a color image forming apparatus, having a plurality of recording units, for forming a color image by superposing images formed with different colors by the respective units.
2. Description of the Related Art
It has been known that in an image forming apparatus, such as a laser-beam printer, for forming an image by scanning an image bearing member with a light beam deflected by a light deflector via a condenser lens, a scanning line does not become a perfect straight line, but always curves.
Accordingly, in a color image forming apparatus using such a laser scanning device, by performing scanning for respective colors using the same optical scanning device (for example, by performing four scanning operations), the same optical scanning characteristics are provided for respective colors, and occurrence of color misregistration is prevented by superposing images having the same distorsion characteristics.
The above-described approach, however, has the problem that in order to obtain a color image, much time is required for outputting images because a plurality of scanning operations must be performed using the same optical scanning device.
In order to solve such a problem, apparatuses for outputting a color image at a high speed by providing a plurality of recording units and providing an optical scanning device for each of the units have been developed.
However, in a method in which an optical scanning device is provided for each of a plurality of recording units, image recording is performed for each color, and images of respective colors are sequentially subjected to multiplex transfer, there is a large possibility of occurrence of color misregistration because respective optical scanning devices have different distortion characteristics.
In order to solve such a problem, there has been found a possibility of not producing optical scanning distortion and therefore preventing occurrence of color misregistration in the plurality of recording units by using solid-state optical scanning devices, each comprising a set of fine recording elements as an optical scanning device, instead of light deflectors.
In such solid-state optical scanning devices, however, since fine recording elements are arranged in a line with an interval corresponding to the recording density, the centers of recording pixels must coincide with one another among the plurality of recording units although optical scanning distortion is not produced.
For that purpose, it is necessary to perform precise position adjustment when assembling respective solid-state optical scanning devices, and to perform temperature control of the entire optical system in order to prevent movement of the center of the recording pixel of each device due to thermal expansion of optical members caused by temperature changes, thereby causing a large increase in the cost of the apparatus.